Steel with high tempering resistance

ABSTRACT

STEEL WITH HIGH TEMPERING RESISTANCE comprising a composition of alloying elements consisting essentially of, in percent by mass, C between 0.20 and 0.50, Si lower than 1.0, P lower than 0.030, Cr between 3.0 and 4.0, Mo between 1.5 and 4.0, V between 0.1 and 2.0, Co lower than 1.5, being the remaining composed of Fe and inevitable deleterious substances. The steel is produced by processes involving ingot casting and hot/cold forming, or used with the cast structure; or by processes involving atomization or dispersion of the molten metal, such as powder metallurgy, powder injection or spray forming.

This report deals with a steel designed for hot metal forming tools,typically in cases where the metal to be formed withstands temperaturesabove 600° C., even though processes at lower temperatures or even atroom temperature can be used with the said steel. The steel in questionhas a composition that allows ranking it as hot work tool steel, whosemain characteristic is increased resistance to loss of hardness at hightemperature—called tempering resistance, while retaining high toughnessand adequate thermal conductivity and hardenability. Such an effect ispossible by carefully designing the alloy, and setting the optimumranges of the elements P, Si, Mo and Cr.

The term hot work tools is applied to a large number of hot-formingoperations, employed in industries and focused on the production ofparts for mechanical applications, especially automotive parts. The mostpopular hot-forming processes are forging, extrusion or casting ofnon-ferrous alloys. Other applications performed at high temperature,typically above 500/600° C., can also be classified as hot work. Inthese applications, molds, dies, punches, inserts and other formingdevices are classified by the generic term: hot work tools. These toolsare usually made of steels, which require special properties towithstand high temperatures and the mechanical characteristics of theprocesses in which they are employed.

Among their key properties of hot work steels, the following stand out:hot resistance, more specifically tempering resistance, toughness,hardenability and physical properties such as thermal conductivity,specific heat, both correlated to thermal diffusivity, and thermalexpansion coefficient.

For forging applications, hot forging of steels stands out, especiallysteels for mechanical construction applied to auto parts. In suchoperations, the forged billet withstands temperatures above 1100° C.During the forming process, it heats up the surface of the tool, giventhat the higher the contact period, the higher the temperature.Consequently, the heat generated requires high hot resistance from thesteel used. The steel hardening mechanism for hot die forming is chieflyinduced by the precipitation of fine carbides. Noteworthy are the Mo orW carbides, M₂C-type, or V carbides, MC-type. For high-Cr steels,Cr-rich M₇C₃ carbides also stand out, but also with Mo and V in solidsolution. Despite their high stability, these carbides tend to coalesceafter long periods of time at high temperatures, typically above 550°C.—conditions easily reached within the tool operating range. As aconsequence, the hardness of the area decreases, causing wear and hotplastic deformation, resulting in tool failure.

Improving the resistance of the material against loss of hardness ortempering resistance, is thus critical to improving the performance ofthe tools that operate under high temperature conditions. Examples ofsuch applications are tools used in hot forging of steel parts or othermetal alloys, extrusion of non-ferrous alloys and dies for castingnon-ferrous alloys (being the latter two applications more important toAl alloys). The same goes for other applications, such as extrusion orcasting of non-ferrous alloys. E.g., in forging steel applications, thetemperature of the pre-forms to be forged is about 1200° C. Evenconsidering the short contact time with the tools (seconds), theirsurface heat up significantly, causing loss of hardness due to temperingof these surfaces. Considering the extrusion of aluminum alloys or othernon-ferrous alloys, the billet temperature is lower, ranging from 400 to600° C. However, in these applications, the contact time issignificantly longer (tens of minutes to hours of operation). Moreover,local friction generated by the tool/aluminum contact intensifiesheating, increasing the loss of hardness of the tool steel and,consequently, leading to steel wear. In pressurized casting dies, themolten metal is injected at high pressure and temperature (around 700°C.), also heating the surface of the die. In this case, failure is duemainly to thermal fatigue cracking caused by the successive heating andcooling of the working surface of the die. But, the high heat exchangebetween molten aluminum and the die surface favors the heating of thesurface areas, generating, as in other applications, loss of hardnessand, consequently, triggering the fatigue cracking process.

This mechanism of post-heating loss of hardness is, therefore,critically important to hot work tool steels; thus, increasing thestrength of the material against this phenomenon is something desirable:Concerning the steel employed, improved hot resistance is usuallyobtained through the use of higher grades of those elements that formsecondary carbides such as Mo, W and V, or by solid solution hardening.Although effective in increasing hot resistance, the excessive increaseof the grade of these elements implies reduced toughness, poor thermaldiffusivity and conductivity or significant increase of productioncosts. This latter economic factor is really important nowadays, giventhe high cost of raw materials used as a source of alloy elements Mo, Wand V.

To provide a better understanding of this invention, we describe belowsome of the state-of-the-art steels currently used (chemical compositionis summarized in Table 1). The H11 and H13 steels stand out, as theseare the tool steels mostly used for hot works. These materials contain5% Cr providing adequate hardenability and to assist with hotresistance, 0.9% V and 1.2% Mo to improve hot resistance, and generallylow grades of P and S to promote adequate toughness. However, forimproving tempering resistance, higher Mo grades would be needed. TheDIN 1.2365 and DIN 1.2367 are steels used for such purpose. They have ahigh Mo grade to improve hot resistance. However, if the content of thiselement is increased within the structure of the DIN 1.2367 steel,toughness and thermal conductivity and diffusivity tend to drop. In the1.2365 steel, this thermal conductivity reduction is counterbalanced byincreasing the Mo content and decreasing the Cr content. However, thelower the Cr content, the lower the hardenability, limiting theapplications in large tools. It is important emphasize that attentionshould be paid to the thermal conductivity and toughness properties.During the work, the increase in thermal conductivity is important suchthat the tool steel is able to homogenize quickly the difference oftemperature between the material formed and the tool core, thus reducingstresses and thermal cracking. And, in case cracking occurs, thetoughness of the material is also critical because it delays propagationand, consequently, thermal fatigue damage. Thus, it is clear that onlyincreasing the Mo content, as exemplified by steel DIN 1.2367 and DIN1.2365, is not sufficient for overall improvement of the properties ofhot work steels.

TABLE 1 Typical chemical composition of state-of-the-art steels. The sumMo + V + Co is shown because these elements have the highest cost, andare closely related to the final cost of the tool steel. Content inpercentage by weight and Fe balance. The sum Mo + W + V + Co is shownbecause these elements impact the alloy cost the most. Mo + W +Designation C Si Cr Mo W V Co V + Co H11 0.36 1.0 5.0 1.2 — 0.5 — 1.7H13 0.38 1.0 5.0 1.2 — 1.0 — 2.2 DIN 1.2365 0.36 0.3 2.8 2.8 — 0.5 — 3.3DIN 1.2367 0.38 0.3 5.0 3.0 — 0.5 — 3.5 PI 9909160-7 0.36 0.2 5.0 2.3 —0.5 — 2.8

A new steel type has been developed more recently and is described in PI9909160-7. This material has, similarly to DIN 1.2367, higher Mocontent, but lower Si and P content to improve toughness. In this case,cost increase is avoided by not using a high Mo content, but the hotresistance gain is not significant in comparison to steel H13.

Given this scenario, it is evident the need for a tool steel with hotresistance higher than that of state-of-the-art steel H13, but withoutusing excessive alloying elements which might affect thermalconductivity and material cost. Also, the material used should featurehigh hardenability, which allows it to be applied to large tools.

Therefore, the steel of the present invention will fulfill all theseneeds.

The initial purpose of the invention was to investigate the influence ofthe Cr and Mo content on hot work tool steels that allowed identifyingsome synergy between the two elements and hot resistance. Morespecifically, when an increase of the Mo content is followed by areduction of the Cr content, a more significant effect on the hotstrength can be observed. In addition, a reduced Cr content improvesthermal conductivity, thereby reversing the negative effect of a higherMo content. On the other hand, the Cr content must be carefullybalanced, because very low values, as previously mentioned, impairhardenability and limit the application to tools with average sectiondimension above 100 mm. Therefore, the material of the present inventionshows a chemical composition with ideal arrangement of the Cr and Mocontents, capable of overcoming the tempering resistance properties ofstate-of-the-art steels, with no significant cost increase and properthermal conductivity, toughness and hardenability.

In order to satisfy the above conditions, the steel of the presentinvention has a composition of alloying elements, which, in percentageby mass consists of:

-   -   0.20 to 0.50 C, preferably 0.3 to 0.45 C, typically 0.36 C    -   3.0 to 4.0 Cr, preferably 3.5 to 3.9 Cr, typically 3.8 Cr    -   1.5 to 4.0 Mo, preferably 2.0 to 3.0 Mo, typically 2.5 Mo. Given        its chemical similarity to W, Mo can be replaced with W, a 2 W:        1 Mo ratio by mass.    -   0.1 to 2.0 V, preferably 0.3 to 1.0 V, typically 0.5 V; V can be        partially or fully replaced with Nb, following a 1.0% Nb: 0.5% V        ratio.    -   up to 1.0 Si, preferably up to 0.5 Si, typically 0.3 Si    -   Max 0.030 P, preferably max 0.015 P, typically max 0.010 P.

Balance by iron and metallic or non-metallic deleterious substancesinevitable to the steelmaking process, in which said non-metallicdeleterious substances include but are not limited to the followingelements, in percentage by mass:

-   -   Max 0.10 S, preferably max 0.020 S, typically max 0.008 S.    -   Max 1.5 Al, Mn or Co, preferably up to 1.0 Al, Mn or Co,        typically below 0.5 Mn, Al and Co.

Next, we describe the ratios of the specification of the composition ofthe new material. The percentages listed refer to percent by mass.

C: Carbon is primarily responsible for martensite hardening under lowtemperature conditions. Together with the alloying elements, carbon actsin the secondary hardening, important for the hardening at hightemperature. For such effects, carbon contents of at least 0.20% arerecommended, preferably above 0.30%. On the other hand, very high Ccontents, cause excessive precipitation of grain-shaped carbides at thetime of quenching (especially when Mo and V contents are high), andresult in increased hardness and volume of secondary carbides. Thus,toughness is generally impaired. the C content should be limited to amaximum value of 0.50%, preferably below 0.40%. This limitation alsoplays a role in the reduction of the amount of retained austenite,preventing problems associated with dimensional instability andembrittlement.

Cr: The chromium content should be higher than 3.0%, preferably greaterthan 3.5%, because this element favors hardenability, which is importantfor application in large tools. However, the Cr content should belimited. The present invention has incorporated the concept of reducingthe Cr content to improve tempering resistance. This is an importanteffect, because the final tempering resistance is higher than that ofstate-of-the-art steels. The mechanisms that cause this effect arethought to be related to the formation of secondary Cr carbides,M₇C₃-type, which dissolve Mo and V are the first carbides to be formed.Therefore, the lower the Cr content, the lower the volume of M₇C₃carbides and, thus, the greater the amount of Mo and V available forsecondary hardening. The end result is a significantly higher temperingresistance when the alloy Cr content is lower than that ofstate-of-the-art alloys. Even in relation to the PI 9909160-7 alloy,there is a significant tempering resistance gain. This is significantbecause the alloys have Mo equivalent grades (Mo is a pricey alloyingelement), showing that the present invention is able to reach high hotresistance values without excessively increasing the Mo content. For allsuch effects, the Cr content should be below the 5.0% content ofconventional steels, being the preferred Cr content lower than 4.0%.Finally, the ideal Cr content required to maximize tempering resistanceidentified in the present invention should be set between 3.0% and 4.0%.In addition to the heat resistance property, a lower Cr content improvesthermal conductivity, also preserving this property as the Mo contentrises. Therefore, this shorter Cr range aims at a careful adjustment formaximum tempering resistance and adequate thermal conductivity.

Mo and W: high Mo contents are used in the alloy of the presentinvention to improve tempering resistance properties. This is possibleby the formation of chemically-stable, Mo-rich tempering carbides,especially the M₂C carbide. Thus, the alloy of the present inventionmust contain at least 1.5%, preferably above 2.0%. On the other hand,excessively high Mo grades can harm toughness due to precipitation ofpro-eutectic carbides at the time of quenching, and may significantlyincrease the cost of the alloy, making its application in many toolsunfeasible. Hence, the Mo content should be limited to 4.0%, preferablybelow 3.0%. Tungsten and molybdenum produce similar effects in the toolsteel of the present invention, forming M₂C or M₆C secondary carbides.Thus, they can be jointly specified through the tungsten equivalentrelationship (W_(eq)) given by the sum W+2Mo, which normalizes thedifferences in atomic weight between the two elements.

V: Vanadium is primarily important for the formation of MC secondarycarbides. Because they are very thin, these carbides block the movementof dislocation lines, increasing mechanical strength. V also improvesgrain growth, allowing high austenitizing temperatures (above 1000° C.).For such effects, V must be above 0.1%, preferably above 0.3%. However,excessively high V grades may generate primary, difficult-to-solubilizecarbides, thus reducing toughness. Hence, the V content should be lowerthan 2.0%, preferably below 1.0%.

Si: silicon produces a strong effect on secondary hardening andtoughness. At high levels, Si increases the secondary hardness up toquenching temperatures of 600° C. However, the study of the presentinvention showed that a lower Si content was important to reduce theloss of hardness under high temperature conditions, thereby increasingtempering resistance. A lower Si content also results in significantincrease of toughness, having this effect been applied to the presentinvention. Therefore, the Si content of the material of the presentinvention must be lower than 1.0%, typically below 0.5%.

P: reduction of the P content also results in significant increase oftoughness, because this element can be segregated on the grain surfaceand, thereby, diminishes cohesion in these surfaces. Therefore, Pcontent should be lower than 0.030%, typically below 0.015%.

Residual Elements: Other elements such as Mn and Al should be consideredas deleterious substances associated with the steelmaking deoxidationprocesses or inherent to the manufacturing processes. Hence, the Mn andAl content should be limited to 1.5%, preferably below 1.0%. The Cocontent should also be limited to the same values, due to its beneficialeffect on hot resistance and strong impact on the alloy cost. In termsof formation of inclusions, the sulfur content should be controlled,because such inclusions may lead to cracking during operation; thereforethe S content should remain below 0.050%, preferably below 0.020%.

The alloy, as described above, can be produced as rolled or forgedproducts through conventional or special processes such as powdermetallurgy, spray forming or continuous casting, such as wire rods,bars, wires, sheets and strips.

The figures attached herein have been referenced to in the descriptionof the experiments carried out, and their contents are listed below:

FIG. 1 shows the effects of P and Si on alloys 1-8, in terms ofpost-tempering toughness and hardness.

FIG. 2 compares alloys 1 to 8, but showing P effect on toughness,depending on the quenching temperature.

FIG. 3 shows the distribution of carbides in the high and low Si contentalloys, demonstrating a better distribution in low Si content alloys,which explains their superior toughness.

FIG. 4 compares the reduction of hardness versus time at 600° C.,showing tempering resistance. The greater the displacement to the right,the higher the alloy's tempering resistance.

FIG. 5 shows a comparison of thermal conductivity values for some of thealloys investigated.

FIG. 6 shows a comparison of toughness of alloys 9 to 12 and alloys IP 1and IP 2; the data produced by the un-notched impact test (7×10 mm testspecimens) and Charpy V

FIG. 7 shows the hot forging punch to which the industrially-produced IP2 steel was applied and compared with the state-of-the-art H13 steel.Note: a) wear failures and cracking; b) hardness profile, showing dropin working areas (distance from surface=zero).

EXAMPLE 1 Effect of Silicon and Phosphorus

Eight experimental ingots were initially produced to evaluate the effectof Si and P on the state-of-the-art H11 steel. The compositions areshown in Table 2. The hardness and impact results are shown in FIG. 1.Note the strong Si influence on hardness for quenching temperaturesbelow 500° C., even though the same effect is not observed for quenchingtemperatures above 600° C.; the hardness of both the alloys with highand low Si content is about the same. The P effect is compared in FIG. 2for different tempering temperatures. In this case, it was possible toobserve that a decreasing P content improves significantly the toughnessof Si-rich alloys, but this effect is less significant on alloys withlow Si content.

Therefore, the results show that the best combination, in terms oftoughness, would be alloys with low P and Si content. Alloys of high Sicontent are only viable for situations when hardness values higher than52 HRC are employed and, for such, tempering temperatures below 600° C.are also adopted. In these cases, the decrease of the P content is evenmore critical.

The reasons for these significant Si and P effects have not been fullydefined, but early scientific results conducted by the inventors of thispatent show a relationship with the formation of secondary carbides. Inalloys with high Si content, the secondary carbides tend to concentratein areas of high diffusion (lath or grain surfaces), because of thedifficulties imposed by Si onto the cementite formation process. On theother hand, in alloys with low Si content, cementite is rapidly formed,leading to a better distribution of secondary carbides formed at highertemperatures. FIG. 3 shows the images of transmission electronmicroscopy that illustrate these observations.

TABLE 2 Chemical composition of the various Si and P contents analyzedfor the state-of-the-art H11 alloy. Alloy: 1 2 3 4 5 6 7 8 C 0.36 0.340.36 0.36 0.36 0.35 0.36 0.35 Si 0.05 0.32 0.98 1.92 0.05 0.33 1.01 1.90Mn 0.35 0.35 0.35 0.35 0.34 0.35 0.35 0.35 P 0.023 0.028 0.024 0.0120.012 0.012 0.011 0.008 S 0.004 0.004 0.004 0.003 0.004 0.004 0.0050.003 Co 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.06 Cr 5.09 5.13 5.06 5.085.08 5.03 5.10 5.05 Mo 1.28 1.31 1.33 1.24 1.32 1.32 1.33 1.23 Ni 0.200.19 0.19 0.20 0.19 0.20 0.19 0.20 V 0.44 0.44 0.42 0.41 0.44 0.44 0.450.43 W 0.10 0.11 0.11 0.10 0.11 0.10 0.11 0.10 Nb <0.01 <0.01 <0.01<0.01 <0.01 <0.01 <0.01 <0.01 Al 0.029 0.020 0.023 0.036 0.024 0.0220.036 0.043 W_(eq) 1.48 1.53 1.55 1.44 1.54 1.52 1.55 1.43 (=W + 2Mo)

EXAMPLE 2 Effect of Cr and Mo

To assist in the definition of the effect of Cr and Mo, seven additionalexperimental ingots have been produced, comprehending fourstate-of-the-art steels: H11, H13 and the steel described in PI9909160-7 and two alloys proposed for the present invention (see Table3). These two compositions lead to a reduction of the Si and P contentsdescribed in example 1 but also to distinct balances of Cr and Mo.

As mentioned, the purpose of the IP 1 and IP 2 alloys is to obtaingreater resistance to loss of hardness, i.e., tempering resistance.Therefore, hardness reduction after different exposure periods at 600°C. was evaluated and the results are shown in Table 4. The timeincrements followed a logarithmic scale, as depicted in the chart ofFIG. 4 b. These results show that, when comparing alloys IP 1 and H11,hot resistance rises simply by modifying the Si and P contents (but theeffect is likely to be related only to Si, since P does not play a rolein the carbide formation process)

However, this gain in hot resistance by reducing the Si content is notsufficient to produce results significantly higher than those of H13.Therefore, a higher Mo content together with a lower Cr content in theIP 2 alloy was used. For that case, a significant variation of the hotresistance could be verified, which provided increased hardness afterthe same exposure period. And, as shown in FIG. 4 a, the same drop inhardness obtained for the H13 steel occurs after far longer periods inthe case of alloy IP 2. For example, hardness reduces from 45 HRc to 35HRc at a temperature of 600° C. after an exposure period of 25 hours,whereas the same phenomenon takes place in alloy IP 2 after only 60hours.

This significant tempering resistance improvement is related not only tothe increase of the Mo content but also to the reduction of the Crcontent. This effect is clear after comparing the differences betweenalloy PI2 and alloy 12 (patent PI 9909160-7). It also explains the hightempering resistance results obtained for alloy 11.

TABLE 3 Chemical composition of state-of-the-art steels and thoseproposed for the present invention. Alloy: 9 10 11 12 IP 1 IP 2 Note:H11 H13 DIN Patent Current 1.2365 9909160-7 invention C 0.36 0.4 0.310.35 0.35 0.35 Si 1.02 0.96 0.3 0.13 0.3 0.31 Mn 0.48 0.34 0.3 0.49 0.270.3 P 0.025 0.023 0.023 0.009 0.007 0.01 S 0.005 0.006 0.005 0.005 0.0050.006 Co 0.02 0.02 0.01 0.02 0.01 0.02 Cr 5.03 5.23 2.85 4.99 4.96 3.78Mo 1.4 1.31 2.8 2.28 1.39 2.49 Ni 0.19 0.20 0.20 0.19 0.20 0.19 V 0.340.85 0.5 0.57 0.42 0.52 W 0.03 0.02 0.02 0.02 0.02 0.02 Nb <0.010 0.020.02 0.01 <0.010 0.005 Al <0.005 0.014 0.019 0.009 <0.005 0.005 W_(eq)2.8 2.6 5.6 4.6 2.8 5.0 (=W + 2Mo) W + Mo + 1.79 2.2 3.33 2.89 1.84 3.05Co + V

TABLE 4 Loss of hardness after exposure at 600° C. for various exposureperiods Initial hardness around 45 HRC Alloy 9 Alloy 10 Alloy 11 Alloy12 IP 1 IP 2 Cr and 5Cr 5.2Cr 2.8Cr 5.0Cr 5.0Cr 3.8Cr Mo %: 1.4Mo 1.3Mo2.8Mo 2.3Mo 1.4Mo 2.5Mo Initial 45.5 44.6 45.3 45.7 44.9 45.4  3 h 43.343.5 44.7 44.1 43.3 44.8  10 h 39.6 41.3 43.8 41.8 40.8 43.7  30 h 34.436.1 41.0 37.6 36.9 40.5 100 h 31.1 30.7 36.0 32.0 32.1 34.5

Despite the interesting effect on tempering resistance, Cr contentsshould not drop to excessively low levels to prevent damaging thehardenability and thus, limiting its application in large tools. Thiscan be considered the major setback of the state-of-the-art DIN 1.2365steel (alloy 11), i.e., excellent tempering resistance but lowhardenability. Table 5 illustrates these Cr×hardenability issues basedon dilatometer test results. The IP 2 composition can be consideredideal under this aspect, with Cr content lower than that of steel H13(alloy 10), to provide increased tempering resistance, but not as low asthat of steel DIN 1.2365 (alloy 11). The higher Mo content of alloy IP 2also helps achieving proper hardenability levels, which compensates theeffect resulting from the Cr content reduction and ensures itsapplication in large tools.

A further advantage of using a lower Cr content than that of alloy 12and other state-of-the art alloys is the ability to maintain adequatethermal conductivity. As shown in FIG. 5, this property tends to fall asthe Mo content increases (compare alloys 12 and 10), and to rise as theCr content goes down (alloys 11 and IP 2). Therefore, in addition tobeing considered ideal with regard to hot resistance, the combination ofthe Cr and Mo contents of alloy IP 2 allows maintaining thermalconductivity at levels even higher than those of the traditional H13steel (alloy 10).

TABLE 5 Results of the TRC curve developed for the investigated steels,used in hardenability evaluation. The lower the critical rate and thehigher the hardness after quenching at 0.1° C., the higher will be thehardenability. Alloy 11 Alloy 12 Alloy 10 DIN PI H13 1.2365 9909160-7IP1 IP 2 Critical rate to start 0.2 8.0 0.3 0.5 0.5 the bainite-formingprocess (° C./s) Hardness after 538 389 534 512 486 quenching at 0.1°C./s (HV)

Superior tenacity is another gain of alloys IP 1 and IP 2 in relation tothe state-of-the-art H11, H13 and DIN 1.2365 alloys (alloys 9 to 11).These results can be compared in FIG. 6. Note the gain of alloy IP 2 inrelation alloy 11, which, similarly, has high tempering resistance. Thatis, in addition to superior hardenability, the balance of the IP 2chemical composition makes this alloy significantly tougher than alloy11. The effect, in this case, is primarily associated with lower Si andP contents, as discussed in example 1.

EXAMPLE 3 Field Test

A field study is detailed next, with allow IP 2 being compared to steelH13 in forging tools. The results were analyzed based on the failuremodes and on the properties of materials.

The process in question deals with high-speed warm forging (see FIG. 7a). Despite the fact that the forged billets are exposed to atemperature lower than the usual hot forging temperature, highprocessing speed makes the contact between the heated billet and thematrix to extend, thus heating its surface. The process is alsodeveloped under high cooling conditions, thereby promoting thermal shockon the surface region.

Process data

Product: shaft end.

Tool: Warm forging precision punch

Forged Material: Modified SAE 1045 and 1050 steels

Billet temperature: approximately 900° C.

Cooling: Intense, water-cooled.

Blow application speed: high.

Tool steel previously used: AISI H13 (hardness: 53 HRC).

Steel tested: IP 2, at same hardness.

FIG. 7 a shows the punch analyzed after the end of its life. Since thistype of forging produces parts of high dimensional accuracy, deviationsof tenths of mm compromise the quality of the part produced. The end ofits life is caused by wear on the protruding and rounded surfaces andoccurrence of thermal cracking (see FIG. 7 a). After the end of itslife, the matrix was destroyed and analyzed. FIG. 7 b shows the dataconcerning hardness vs. distance from the contact surface; note hardnessdecrease close to subsurface areas. Wear is actually related to thisloss of hardness during the work, regardless of core hardness. Theoccurrence of thermal cracking is also associated with loss of hardness,as surfaces of lower hardness become more sensitive to the occurrence ofthermal cracking. Therefore, increased tempering resistance is essentialto further increasing the tool's life time.

The steel of the present invention, IP 2, was then tested and approvedfor the application, increasing the tools life time by 56%. In numericalvalues, 5000 parts made of H13 steel could be forged up to the end ofthe tool's life and this figure increased to 7500 parts made of steel IP2; the comparative analysis of the tempering curves and hardness vs.time developed for steels H13 (alloy 10) and IP 2, FIGS. 4 and 6,provides a better understanding of the phenomenon. For both cases thesteel hardness decreases when subjected to high temperatures, thegreater the drop, the longer the time and the higher the temperatureemployed. However, there is greater stability of alloy IP 2 at hightemperature. Thus, during the forging process, the failure will occurafter a higher number of strokes, resulting in the yield gain that wasobserved.

1-10. (canceled)
 11. STEEL WITH HIGH TEMPERING RESISTANCE comprising acomposition of alloying elements consisting essentially of, inpercentage by mass, C between 0.20 and 0.50, Si at maximum 0.30, P lowerthan 0.030, Cr between 3.0 and 4.0, Mo between 1.5 and 4.0, V between0.1 and 2.0, Co lower than 1.5, being the remaining composed of Fe andinevitable deleterious substances.
 12. STEEL WITH HIGH TEMPERINGRESISTANCE, according to claim 11, comprising a composition of alloyingelements consisting essentially of, in percentage by mass, C between0.30 and 0.50, Si at maximum 0.30, P lower than 0.020, Cr between 3.0and 4.0, Mo between 2.0 and 3.0, V between 0.1 and 1.0, Co lower than1.0, being the remaining composed of Fe and inevitable deleterioussubstances.
 13. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim12, comprising a composition of alloying elements consisting essentiallyof, in percentage by mass, C between 0.30 and 0.45, Si at maximum 0.30,P lower than 0.015, Cr between 3.2 and 3.9, Mo between 2.0 and 3.0, Vbetween 0.3 and 1.0, Co lower than 1.0, being the remaining composed ofFe and inevitable deleterious substances.
 14. STEEL WITH HIGH TEMPERINGRESISTANCE, according to claim 13, comprising a composition of alloyingelements consisting essentially of, in percentage by mass, C between0.30 and 0.40, Si at maximum 0.30, P lower than 0.010, Cr between 3.5and 3.9, Mo between 2.2 and 2.8, V between 0.3 and 0.8, Co lower than0.5, being the remaining composed of Fe and inevitable deleterioussubstances.
 15. STEEL WITH HIGH TEMPERING RESISTANCE according to claim11, wherein a Mo/W replacement ratio is 1 Mo:2 W.
 16. STEEL WITH HIGHTEMPERING RESISTANCE according to claim 11, wherein a Vanadium:Niobium/Titanium replacement ratio is 1 V:2 Nb/1 Ti.
 17. USE OF STEELWITH HIGH TEMPERING RESISTANCE according to claim 11, for molds, diesand multiple-use tools, for formation of solid or liquid materials, atroom temperature or at temperatures up to 1300° C.
 18. USE OF STEEL HIGHRESISTANCE TEMPERING according to claim 11, for metal-forming toolssubject to temperatures between 300 and 1300° C., and also to otherapplications such as forging, extrusion or casting ferrous or nonferrousalloys.